Many surface modification processes involve addition or subtraction of a layer or material on or from a specimen based on certain design, formula, recipe or physical pattern. Surface modification is used for a variety of purposes, for example, surface modification can be used to achieve desired optical, mechanical, electrical, photovoltaic or any other physical feature that influences performance. Surface properties such as roughness, relief, chemical homogeneity, uniformity and optical, electrical, and mechanical properties can influence the properties or performance of the final device. Surface modification processes have a wide range of applications including but not limited to semiconductor fabrication, solar cells, optical thin films, nanotechnology, printing, etc.
One feature of the surface modification processes is that they typically involve repetition of the same product design or the product recipe multiple times. For example, in the printing process, all printed copies have to be the same within certain acceptable tolerances. In the manufacturing of solar cells, thin films, microelectronics, polishing, and many other processes which require surface modification it is expected that all manufactured products will be the same according to the initial product design within some accepted tolerances.
Thin film process is one example of an additive surface modification process. Thin films are thin material layers ranging from fractions of a nanometer to several micrometers in thickness. Thin films are formed (ex. by deposition) on a bare specimen or over already existing features of a device. The typical thin film deposition process takes place in vacuum conditions, which are created in a vacuum chamber. The process usually consists of creating vapors of material by chemical of physical means such as evaporation, sputtering, plasma, and subsequent condensation on the vapors on the surface of the deposited specimen or device. However, many varieties of thin film deposition processes exist, such as plating, printing, spraying, thermal diffusion, electro-chemical deposition, surface oxidation, wet deposition by spinning or dipping, formation of thin films from aqueous solutions, etc.
Thin film formation is a complex process requiring thorough control of the process parameters and in some cases, control of film characteristics, such as optical, electrical, thermal properties and mechanical stresses in the film, etc, while maintaining geometrically, stoichiometrically and structurally uniform films. Some of the materials that have been used to form thin films include amorphous silicon, crystalline silicon, oxides, nitrides, a variety of semiconductors, dielectrics, metals, polymers, inks, toners and others. Thin-films are often deposited in multiple layers to generate the specifications as desired by the manufacturer. In some cases, there is no definitive interface between separate layers as their properties gradually change from one layer to another. In other cases, the thickness and the properties of the layers are modulated, or vary in certain pre-designed ways in depth of the coating. Furthermore, the thickness and the properties of the layers can also be modulated or vary in all 2 or 3 dimensions as is the case of variety of patterned coatings, subtractive color synthesis (color printing), thin film microlenses, photonic crystals, waveguides, optical displays, and other optical products. A large variety of substrates can be used, including flexible substrates and substrates that can later be removed or etched away leaving the thin film coating to be self-supported, or to be transferred to another substrate.
One example of a subtractive surface modification process is the surface etching. Etching is used in micro-fabrication to remove layers from the surface of a specimen (e.g., a wafer) during manufacturing. Another example of subtractive surface modification process is the layer removal by laser ablation or mechanical scribing process.
Etching, scribing, and ablation are very precise processes which require very strict control of the process parameters in order to achieve the desired etching rate and selectivity. In the ion etching process, control of the ratio of ion/reactive components in the plasma offers a convenient means to control the etching rate and etching profile. Another convenient means to control the process is achieved by applying bias voltages with different magnitude, profiles, waveforms, etc.
For manufacturing of typical semiconductor elements (such as thin film transistors in display technology or semiconductor chip in microelectronics) every wafer undergoes many deposition and etching steps one after another in a pre-designed fashion. For many etch steps, part of the wafer is protected from the etchant by a “masking” material which resists etching. In some cases, the masking material is a photoresist which has been patterned using photolithography. Other situations require a more durable mask, such as patterned silicon nitride layer deposited on the wafer or over the thin film structure already deposited on the wafer. Yet, in some cases patterns are formed on the specimen by deposition of thin film through a mask located in front of the specimen in contact with the specimen (contact mask) or at a distance from the specimen (shadow mask). Examples of these are manufacturing of some thin film micro lenses, some photonic crystal structures and nanostructures.
Other examples of surface modification processes include surface grinding/polishing, implantation, ablation, printing, spraying, diffusion of material through the surface, surface wear, etc.
Most surface modification processes typically tend to “drift” over time, causing the modified surface or specimen to gradually deviate from the target values.
In the case of printing, a residual deposition of ink on the printing drums, variations in the viscosity of the printing inks, sublimation of thermoplastic resin or formation of toner particle layers on the rollers and drums in electro-photography, can cause gradual drift in the quality of the produced copies over time, and from location to location on the same sample.
In the case of surface grinding/polishing, a removal of material from the surface of the specimen and its subsequent incorporation into the polishing solution may gradually change the chemical properties of the polishing solution (such as its pH) causing formation of unwanted substances on the surface and gradual drift in the quality of the polished samples.
In the typical thin film deposition, one reason for the process to “drift” over time can be the overcoat of the processing chamber walls during the process, causing change in the thermal, optical or electrical properties inside the chamber. For example, deposition of a dielectric layer on the chamber walls during the process may gradually change the electrical conductivity and/or electro-isolation properties of the process surrounding area, the reflective properties of the walls or create temperature gradients which gradually affect the quality of the deposited specimens. In addition, there is a spatial distribution of the processing parameters inside the equipment due to geometrical or other reasons, causing non-uniformity in the plasma or gaseous phase distribution and, thus, non-uniformity of the film properties over the deposited specimen from location to location. To further complicate the situation, this spatial parameter distribution inside the processing chamber can also drift over time.
Typical example of drift in the process parameters during the process are the wear off of the sputtering target as material is removed from it, which changes the spatial distribution of the material. Another example is the decrease of evaporated material in the thermal boat as it evaporates, which may cause a gradual increase of the boat temperature and therefore, the deposition rate and the kinetic energy of the evaporated particles.
Another reason for the drift in the thin film process parameters can be the change in the temperature, pressure, current or another parameter due to the fact that the measurement sensors can change during the process. For example, deposition of material on a thermocouple changes its thermal capacity and, therefore, the temperature reading. Similar could be the situation with other sensors such as vacuum meters, gas flow meters, etc.
One example of thin film parameter drift is associated with the deposition of multi-junction solar cells. Multi junction solar cells can make better use of the solar spectrum by having multiple semiconductor layers with different band gaps. Triple junction solar cells currently in production are made of GaInP, GaAs, and Ge, which have band gaps of 1.8 eV, 1.4 eV, and 0.7 eV, respectively. In the multi junction solar cells, the different semiconductor layers are epitaxially grown directly on top of the other layers using the same substrate. As a result of this method, the lattice constant, which describes the spacing between atoms of a crystal structure, must be the same for all of the layers. A lattice mismatch small as a fraction of a percent can significantly affect the career mobility and decrease the current produced by the solar cell. Even a very small change in any of the process parameters can cause a sufficient lattice mismatch and, thus, reduce the efficiency of the final product, making it to miss its product specification.
Many modifications are made in order to partially or completely mitigate the effect of process parameters drift during the typical thin film formation processes. In some ion assisted deposition processes, the growing film is bombarded with inert or reactive ions and accelerated particles in order to supply additional kinetic energy to the surface to enhance the surface mobility of the deposited particles and thus facilitate better film growth. Another widely used solution to the problem of parameter drift is the application of different bias voltages which can modify the particle distribution during deposition or etching and enhance the formation of the film or the etched profile regardless of the other parameters' drift.
The traditional state-of-the-art process control systems and methods usually integrate over time and/or space the measured process parameter values (such as temperature, pressure, current density, bias voltage, gas flow, etc.) and try to keep them constant or within certain tolerances. In many cases, specifically in optical, semiconductor, and photovoltaic thin film processes as well as in printing, polishing, implantation, and others there is no real time monitoring of the surface under modification and decision process based on what is really taking place on the specimen. The state-of-the-art control means typically “judge” the product on a “pass/fail”, step-by-step or run-to-run basis. Errors in the manufactured product are discovered too late to be corrected for the failed sample and can be corrected only for the next sample. When the products fail outside the acceptable tolerances they are rejected and the process controls are modified for the next batch of the next product. One result of this fact is that there is unavoidable percentage of rejects or final products, which are not able to meet the intended product design.
Different control schemes are devised to address the non-uniformity and parameter drift problems. Sometimes, to overcome the problem, the technologists and manufacturing engineers are forced to initially “over-design” the product to ensure that even with process drifts the final specification would still be achieved with acceptable manufacturing yield. Run-to-run control, feedback control, fault detection control and like, all intend to reduce the non-uniformity and increase the efficiency of manufacturing by measuring the outcome of the process “post factum” and correcting the process for the next sample, next run or next batch. As for the flawed sample, it is usually considered a reject or a product with inferior quality. This results in wasted materials, energy and labor and inflates the final product cost.
FIG. 1 shows a prior art state-of-art run-to-run process control, widely used in the semiconductor and other thin film manufacturing today.
From a known product specification 100 an initial design or appropriate model is chosen 101 and a series of sub-steps (recipe) is developed or generated. The recipe is loaded into the equipment control system 102 together with other additional parameters or equipment constants 103 needed to run the process such as tooling factors, calibration constants, etc. The manufacturing process starts with executing the parameters for the first manufacturing step 105. After completion of the step (or several steps one after another), the specimen can be measured/tested 106 in order to make decisions 107 about its intermediate quality by comparing it to an intermediate target 104, which is predetermined during the product design or product recipe step 101. If the intermediate target is not met within the accepted tolerance, the specimen is rejected 108 and the process parameters for the next specimen are changed 109 to correct the variance and meet the intermediate target 104 whiting the acceptable tolerance. The next decision point 110 involves a decision regarding whether all the manufacturing steps are already completed. If all steps are not completed, the specimen is sent for the next manufacturing step. After completion of all steps, a final inspection 111 is performed as to find out whether the initial product design is met. In many cases the inspection also involves comparing the product qualities with the initial specification 112. If the product design of the product specification is not met the product may be rejected or sold as inferior product 113 and the product design, model or process recipe are updated 114 in order to correct for the next product or batch of products. If the product specification is met the manufacturing process ends 115.
It is important to emphasize that the main goals of the prior art process control is typically the achievement of the initial product design within tighter tolerances. The process parameters or the recipe may change from product to product, but the initial product design/model is typically fixed at the beginning of the manufacturing process and remains static during the manufacturing. Typically there is no correction process that would correct the product design for each specimen in order to reflect its individual development during manufacturing and adaptively return it to its intended specification in case of deviation.
As a result, there is a need in the art for better methods and systems for controlling variance during surface modification processes that would allow real-time correction of the faulty specimen and its return to the intended specification. The present invention provides for real time detection of error and/or variance during surface modification processes and real time change and re-optimization of the initial product design or model in order to achieve desired product specification, for each specimen under manufacturing.